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SP6685 Datasheet(PDF) 12 Page - Exar Corporation |
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SP6685 Datasheet(HTML) 12 Page - Exar Corporation |
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12 / 14 page  S SP P6 66 68 85 5 7 70 00 0m mA A B Bu uc ck k/ /B Bo oo os stt C Ch ha arrg ge e P Pu um mp p L LE ED D D Drriiv ve err © 2009 Exar Corporation 12/14 Rev. 2.0.0 RESISTOR SELECTION The sense resistor RSENSE is determined by the value needed in the Torch mode for the desired output current by the equation: RSENSE = VFB/IOUT Where VFB = 50mV (Torch Mode) Once the RSENSE resistor has been selected for Torch mode, the VFB voltage can be selected for Flash mode using the following equation: VFB = IOUT * RSENSE (Flash Mode) Where IOUT is for Flash Mode. Next, the RSET resistor can be selected for Flash mode using the following equation: RSET = (1.26V/ VFB)*11.2KΩ (Flash Mode) For an example of 200mA Torch mode and 600mA Flash mode, the values RSENSE = 0.25Ω, VFB = 150mV (Flash Mode), and RSET = 94KΩ are calculated. The power obtained in the Flash mode would be: PFLASH = VFB *IOUT = 150mV*600mA = 90mW. The typical 0603 surface mount resistor is rated 1/10 Watt continuous power and 1/5 Watt pulsed power, more than enough for this application. For other applications, the PFLASH power can be calculated and resistor size selected. The RSENSE resistor is recommended to be size 0603 for most applications. The range of typical resistor values and sizes are shown in table 2. Part Ref. Value Tolerance Size RSET 68KΩ 5% 0402 RSET 75KΩ 5% 0402 RSET 82KΩ 5% 0402 RSET 91KΩ 5% 0402 RSET 100KΩ 5% 0402 RSET 110KΩ 5% 0402 RSET 120KΩ 5% 0402 RSET 130KΩ 5% 0402 RSET 140KΩ 5% 0402 RSET 150KΩ 5% 0402 RSENSE 0.22Ω 5% 0603 RSENSE 0.27Ω 5% 0603 RSENSE 0.33Ω 5% 0603 RSENSE 0.39Ω 5% 0603 RSENSE 0.47Ω 5% 0603 Table 2: Resistor Value and Sizes EVALUATION BOARD CIRCUIT LAYOUT |
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